Certain types of communication channels are generally restricted in the amount of data that are included in each transmission and in the characters that can be transmitted. If the message is short and limited to the characters that can be transmitted over the channel, the limitations do not present a problem. For example, the accepted protocol for paging channels used to transmit signals to portable paging devices limits the amount of data in each transmission to 240 characters. In addition, only printable American National Standard Code for Information Interchange (ASCII) characters can be transmitted over a paging channel. Since conventional pagers typically have displays on which only a few lines of an alphanumeric message comprising only the printable characters can be displayed, this limitation has not previously posed much of a problem. However, new applications for paging and other portable electronic communication devices are being developed that will greatly expand the amount and type of data, which will be transmitted over paging and other communication channels, and as a consequence, such limitations have become a problem.
The problem is most evident in cases where data not limited to printable ASCII characters must be transmitted over a paging channel or other similarly limited communication channel. Such data may include any of the 256 characters in the expanded ASCII character set typically used in many fonts. The data will be transmitted to a new type of page receiver that includes a microprocessor with a limited memory. In addition to receiving conventional paging messages, the pager also serves as a personal information management (PIM) device. The microprocessor in the pager can execute small software applications called "applets." The data transmitted to this new pager thus often include ASCII characters that are not within the limited printable ASCII character set specified for transmission over paging channels.
One solution to this problem would be to encode the entire data to be transmitted using only the printable ASCII characters that are allowed to be transmitted over the pager communication channel. Such an approach would thus be similar to that used in the UUEncoding of binary files into E-mail messages for transmission over the Internet. However, the technique will often produce an encoded data file that is substantially larger than the original data file, and the encoded file size that is thus produced can easily exceed the 240 characters per transmission limit set by the paging industry. Accordingly, it will be evident that a modified approach must be employed to encode full range of ASCII characters in the data that must be transmitted to the new pager using only the limited printable ASCII characters, so that the resulting encoded data transmitted are compact and the encoded data are less than the limit imposed by the paging channel specification. Furthermore, the method must impose a relatively minimal processing load on the microprocessor included in the pager, so that its efficiency in handling all of the other functions it provides is not adversely affected.